3GPP Long Term Evolution (3GPP LTE)
Third-generation mobile systems (3G) based on WCDMA radio-access technology, such as UMTS (Universal Mobile Communications System), are currently deployed on a broad scale all around the world. A first step in enhancing or evolving this technology entails introducing High-Speed Downlink Packet Access (HSDPA) and an enhanced uplink, also referred to as High Speed Uplink Packet Access (HSUPA), giving a radio-access technology that is highly competitive.
In order to be prepared for further increasing user demands and to be competitive against new radio access technologies 3GPP introduced a new mobile communication system which is called Long Term Evolution (LTE). LTE is designed to meet the carrier needs for high speed data and media transport as well as high capacity voice support to the next decade. The ability to provide high bit rates is a key measure for LTE. The work item (WI) specification on LTE called Evolved UMTS Terrestrial Radio Access (UTRA) and UMTS Terrestrial Radio Access Network (UTRAN) is to be finalized as Release 8 (LTE Rel. 8). The LTE system represents efficient packet-based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The detailed system requirements are given in 3GPP TR 25.913, “Requirements for Evolved UTRA (E-UTRA) and Evolved UTRAN (E-UTRAN),” version 8.0.0, January 2009 (available at http://www.3gpp.org and incorporated herein by reference).
In LTE, scalable multiple transmission bandwidths are specified such as 1.4, 3.0, 5.0, 10.0, 15.0, and 20.0 MHz, in order to achieve flexible system deployment using a given spectrum. In the downlink, Orthogonal Frequency Division Multiplexing (OFDM) based radio access was adopted because of its inherent immunity to multipath interference (MPI) due to a low symbol rate, the use of a cyclic prefix (CP), and its affinity to different transmission bandwidth arrangements. Single-carrier frequency division multiple access (SC-FDMA) based radio access was adopted in the uplink, since provisioning of wide area coverage was prioritized over improvement in the peak data rate considering the restricted transmission power of the user equipment (user equipment). Many key packet radio access techniques are employed including multiple-input multiple-output (MIMO) channel transmission techniques, and a highly efficient control signaling structure is achieved in LTE Rel. 8.
Packet-Scheduling and Shared Channel Transmission
In modern wireless communication systems employing packet-scheduling, at least part of the air-interface resources are assigned dynamically to different receivers. In a communication system, typically the packet-scheduling is done by a network node (typically the Node B or base station), and the receivers are usually terminals or user equipments (UE). In advanced communication systems, it is also possible to employ so-called “Relay Nodes” (RN) or “relays” that act as an intermediate transceiving node between the Node B and the user equipment. Since the relay node is connected to the Node B in the same way as a user equipment, the Node B has to allocate resources to the user equipments as well as to the relay nodes within its reach.
From the perspective of making the resource allocation known within the communication system, the Node B (sometimes also called eNB or eNode B) can be seen as a transmitter, while relay node and user equipment act as receivers. Depending on whether the resource allocation actually assigns transmissions or receptions, any of the Node B, relay node, or user equipment can act as either a transmitter or receiver, as will be appreciated by those skilled in the art. In the following, the role of “transmitter” and “receiver” is assumed to be with respect to the described scenario for making the resource allocation known. In short, this is achieved by the Node B transmitting a control channel, that carries the resource allocation information, and which is received by relay node and user equipment.
The dynamically allocated resources are usually mapped onto at least one SDCH (Shared Data CHannel), where a SDCH corresponds to e.g. the following configurations:                One or multiple codes in a CDM(A) (Code Division Multiple Access) system are dynamically shared between multiple MS.        One or multiple subcarriers (subbands) in an OFDM(A) system are dynamically shared between multiple MS.        Combinations of the above in an OFCDM(A) (Orthogonal Frequency Code Division Multiplex Access) or a MC-CDM(A) (Multi Carrier-Code Division Multiple Access) system are dynamically shared between multiple MS.        
FIG. 1 shows a packet-scheduling system on a shared channel for systems with a single SDCH. A sub-frame reflects the smallest interval at which the scheduler (PHY/MAC Scheduler) performs the DRA (Dynamic Resource Allocation). Further, typically the smallest unit, which can be allocated, is defined by one sub-frame in time domain and by one code/subcarrier/subband in code/frequency domain. In the following, this unit is denoted as PRB (Physical Resource Block). Note that the DRA is performed in time domain and in code/frequency domain.
The main benefits of packet-scheduling are as follows:                Multireceiver diversity gain by TDS (Time Domain Scheduling): Assuming that the channel conditions of at least some receivers change over time due to fast (and slow) fading, at a given time instant the scheduler can assign available resources (codes in case of CDM, subcarriers/subbands in case of OFDM) to receivers having good channel conditions        Dynamic receiver rate adaptation: Assuming that the required data rates by the receivers (services a receiver is running) changes dynamically over time, the scheduler can dynamically change the amount of allocated resources per receiver.        
L1/L2 Control Signaling
In order to inform the receivers about their resource allocation, assigned transmission transport format and other data related information (e.g. HARQ), L1/L2 control signaling needs to be transmitted to the receivers. The control signaling needs to be multiplexed with data in a sub-frame (assuming that the allocation can change from sub-frame to sub-frame). Here, it should be noted, that the allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length is a multiple of the sub-frames. The TTI length may be fixed in a service area for all receivers, may be different for different receivers, or may even by dynamic for each receiver. Generally, then the L1/2 control signaling needs only be transmitted once per TTI, however, in some cases it may make sense to repeat the L1/2 control signaling within a TTI in order to increase the reliability. The following description focuses on a constant TTI length of one sub-frame, however, it is equally applicable to the various TTI configurations described above.
In 3GPP LTE Release 8, the L1/L2 control signaling is multiplexed with SDCH in a TDM fashion, such that the L1/L2 control signaling is transmitted in an early part of a sub-frame, while the SDCH is transmitted in the (remaining) late part of a sub-frame.
L1/L2 Control Channel Transmission in 3GPP LTE Release 8
The PDCCH carries one or more messages known as Downlink Control Information (DCI), where each DCI is equivalent to a L1/L2 Control Channel message. It should be noted that the terminology “downlink control information” relates only that control information is sent on the downlink. However, the message it contains can represent either a downlink or an uplink resource assignment/allocation and/or other content.
Each PDCCH is transmitted using one or more so-called Control Channel Elements (CCEs), where each CCE corresponds to nine sets of four physical resource elements known as Resource Element Groups (REGs). The CCEs are all transmitted within the Control Channel (CCH) Region as shown for example in FIG. 8.
The number of CCEs used for a particular PDCCH is determined according to the channel conditions. Generally each receiver has to check the whole control channel region to identify if any DCI is addressed (i.e. directed) towards it.
TD Relay
For the relay functionality, it is first assumed a layout as exemplary shown in FIG. 2. The Node B transmits L1/L2 control and data to a so-called macro-user equipment (UE1) and also to a relay (relay node), and the relay node transmits L1/L2 control and data to a so-called relay-user equipment (UE2).
Further assuming that the relay node operates in a time-duplexing mode, i.e. transmission and reception operation are not performed at the same time, we arrive at a non-exhaustive entity behavior over time as shown in FIG. 3. Whenever the relay node is in “transmit” mode, UE2 needs to receive the L1/L2 control channel and SDCH, while when the relay node is in “receive” mode, i.e. it is receiving L1/L2 control channel and SDCH from the Node B, it cannot transmit to UE2 and therefore UE2 cannot receive any information from the relay node in such a sub-frame.
The situation becomes somewhat trickier in case that the UE2 is not aware that it is attached to a relay node. As will be understood by those skilled in the art, in a communication system without relay node any user equipment can always assume that at least the L1/L2 control signal is present in every sub-frame.
In order to support such a user equipment in operation beneath a relay node, the relay node should therefore pretend such an expected behavior in all sub-frames. This leads to a behavior as shown in FIG. 4. The relay node has to transmit the L1/L2 control channel in each sub-frame (here assumed to be in the early part of each sub-frame), before it can switch to reception mode. Additionally shown is a “Gap” which is required to tune the relay node hardware and software from “transmit” to “receive” mode and vice versa, which is typically a fraction of a sub-frame. What can be seen is that effectively the time that is available for transmission from a Node B to a relay node is actually only a fraction of a sub-frame, as indicated in the figure by the dashed box. In 3GPP Release 8, the UE2 behavior shown for sub-frame 2, i.e. to receive only the first part identical to the L1/L2 control signaling, can be achieved by configuring that sub-frame as an “MBSFN sub-frame”. Since this is done mainly to tell the UE2 to not process or expect the remainder of that sub-frame, it is also sometimes called a “fake MBSFN sub-frame”. In LTE, a node transmitting such “fake MBSFN” sub-frames is required to transmit the first two OFDM symbols of such a sub-frame before it can switch to reception.
Propagation Delay Between Node B and Relay Node
As shown in FIG. 5, we can usually assume that more than a single relay node is deployed and connected to a Node B. In addition, it is possible that the relay node is not stationary, but can be mobile as a user equipment. For example, a relay node can be installed in a public transportation vehicle such as a bus, train, or tramway. In any case, the distance between Node B and at least one relay node is variable, so that different propagation delay for the signal from Node B to relay nodes will occur.
Using the exemplary deployment of FIG. 5, FIG. 6 illustrates the situation assuming that the relay nodes' transmission is synchronized to the Node B's transmission, as it is for example beneficial for the case that a user equipment should easily hand over between the Node B and a relay node or for simultaneous multipoint transmission purposes. For the first two OFDM symbols of the fake MBSFN sub-frame, Node B, RN1, and RN2 transmit simultaneously. Then for the relay nodes the first gap is required to switch to reception mode, followed by reception of the Node B transmission signal until just before the end of the sub-frame, where the second gap is required by the relay nodes to switch back again to transmission mode before the beginning of the next sub-frame.
As can be seen, depending on the length of the gaps and propagation delay for the signal between Node B and RN1 and between Node B and RN2; a relay node will be able to see only a limited and at least partially different set of OFDM symbols transmitted by the Node B. For RN1, the reception of OFDM symbol #1 overlaps with the gap, as does the reception of OFDM symbol #12. For RN2, the reception of OFDM symbol #2 overlaps with the gap, as does the reception of OFDM symbol #13. While RN1 can see OFDM symbols #2 to #11 completely, RN2 can see OFDM symbols #3 to #12 completely. Assuming a simple and cost-effective receiver at the relay node, partially invisible OFDM symbols cannot be used since they would contain a lot of interference and should therefore be considered as corrupt.
As seen from FIG. 4, the relay node is not able to detect the early part of a sub-frame transmitted by a Node B, which usually carries L1/L2 control information. Therefore a new method must be devised how to convey L1/L2 control signaling from Node B to relay node. In addition, different relay node nodes will be able to see different OFDM symbols from the Node B, such that provision should be taken that the L1/L2 control information is transmitted such that all relay nodes attached to a Node B are able to detect and receive that information.